1. Field of the Invention
The present invention relates to a control device and a control method for an automatic transmission for vehicle and particularly to a device and method for controlling the working oil pressure of a hydraulic clutch (lockup clutch) which mechanically and directly engages an input shaft with an output shaft of a torque converter.
2. Related Art of the Invention
It is well known that the transmission efficiency of a torque converter can be improved by providing a hydraulic lockup clutch which mechanically and directly engages an input shaft with an output shaft of a fluid type torque converter and by locking up the above lockup clutch given a certain condition in a conventional automatic transmission of a vehicle.
During lockup control of the lockup clutch as mentioned above, if the lockup clutch is fully and directly engaged at low speed, when drive train vibration is large, a difference in rotation speeds between the input shaft and the output shaft in the torque converter becomes almost zero. Thus, at low speed, vibration and noise are increased, which is a problem, so that full lockup of the clutch is limited to the high speed side where drive train vibration is less.
In some conventional examples, semi-clutch control (slip lockup control) has been executed on the low speed side, where the drive train vibration is relatively large, by carrying out feedback control of a working oil pressure to be supplied to the lockup clutch so that the difference in rotation speeds between the input shaft and the output shaft of the torque converter becomes a predetermined value. When the above semi-clutch control is executed, efficiency of the torque converter can be improved even on the low speed side as a consequences of the drive train vibration being absorbed by the torque converter.
Also, when the difference in rotation speeds between the input shaft and the output shaft of the torque converter is obtained as mentioned above and the supply hydraulic pressure for the lockup clutch is feedback controlled so that this difference is converged into the neighborhood of zero, the working oil pressure in the full lockup control can be optimized according to input torque of the torque converter (See Japanese Unexamined Patent Publication No. 2-38754).
When lockup of the lockup clutch is to be controlled based on the difference in rotation speeds between the input shaft and the output shaft of the torque converter, engine revolution speed calculated using a detection pulse signal of a crank angle sensor can be used to determine the rotation speed of the input shaft of the torque converter, since the rotation speed of the input shaft is equal to the revolution speed of the engine. In the meantime, the rotation speed of the output shaft can be detected based on car speed and transmission gear ratio calculated from a detection pulse signal of a car speed sensor which obtains a rotation signal from an output shaft of the transmission.
Rotation speed can be determined using sensors which generate pulse signals synchronously with rotation, such as a crank angle sensor and a car speed sensor. The rotation speed may then calculated by obtaining a pulse generation cycle or by counting the number of generated pulses per unit time.
Higher accuracy is generally required for detecting the engine revolution speed crank angle sensor usually has a resolution higher than that of the car speed sensor, meaning that the engine revolution speed Ne is renewed in a shorter cycle.
Thus, the engine revolution speed Ne and a car speed VSP are different in an updating cycle of a rotation speed detected value, and in calculating the difference in rotation speeds of the both, the calculated difference in rotation speeds might be hunting due to a gap in updating timing of each of the rotation speeds regardless of an actual rotation difference, which in turn will lead to an error. This presents a problem since it is hard to execute lockup force control of the lockup clutch with high accuracy based on the difference in rotation speeds.
That is, with a rotation sensor, an updating cycle of calculated rotation speed is governed by the resolution of the sensor, and when the resolution of a sensor is low, a longer updating cycle is required. Then, if a difference in rotation speeds detected by rotation sensors with different resolution is to be obtained, a desired rotation speed difference can not be obtained due to a gap in updating timing, and when the updating timing is to be adjusted, such a problem occurs that updating at high speed becomes impossible or that calculating accuracy deteriorates. Therefore, it has been hard to calculate the difference in rotation speeds between the input and the output shafts of the torque converter at high speed and with high accuracy and it has been impossible to carry out lockup control with high accuracy based on the above difference in rotation speeds.